well the cost is all depended on how advanced of technology you want (SAS expander vs Sata) and how much space you want. This build after the drives are bought is probably in the 3k+ price range. But then again i built a 24 drive SAS backplaned monster with 15TB of usable space in3 raid units. You may not need 15 TB. The speed is quite nice. Put it this way. Write performance on raid 10 was around 200MB/s sustained on the server. since gigabit basically maxes out at 125MB/s when using jumbo frames your disk IO will never be the limiting factor (unless you put in 10gbe). I have since reconfigured my array into a raid 6 volume and will post new benchmarks. The first step would be in deciding how much space you need. Here are some examples of what i would do. in all cases i am going to be using a raid card since i like the speed and security of hardware raid.

Small File Server. (under $1500)

Raid Card: 3ware 9690SA-4i or 9650 or Perc5i/6i
Raid Level: 5
Disks: 1TB or 2TB WD Blacks (greens can cause issues since they power down when not in use)
Usable space - 3TB using 1TB disks and 6TB using 2TB disks, Failure of 1 disk is allowed before array fails

Medium File Server (under $2200)

raid Card: 3ware 9690SA-8i or Perc6i
Raid Level 6
Disks: 1TB or 2TB WD Blacks (greens can cause issues since they power down when not in use)
Usable Space: 6TB using 1Tb disks, 12TB using 2TB disks

Large File Server ($3k+)

Raid Card: 3ware 9690SA-4i or any other RAID SAS Controller
Raid Level: 6
Disks: 1TB or 2TB WD Blacks (greens can cause issues since they power down when not in use)
Usable Space: 6TB using 1TB drives per array, 12TB of space using 2TB disks per array.

The pricing is rough ballparks they could be more or less depending on hard selections. obviously buying a used perc is alot chepaer than a brand new raid controller. and 1TB drives are considerably cheaper than 2Tb drives.

anyway back to the benchmarks. These are all read tests. Lots of bandwidth.